Linker Engineering in β-Ketoenamine-Based Covalent Organic Frameworks for Photocatalytic Water Splitting

Covalent organic frameworks (COFs), composed of knot and linker organic moieties that are connected via covalent bonds, have emerged as promising metal-free photocatalysts with tunable electronic properties. However, precise tuning of the electronic properties of COFs for modulating the energy level positions of the overall photocatalytic water splitting is crucial. In this study, we have used first-principles calculations with the state-of-the-art level of theory to investigate a series of β-ketoenamine-based COFs, exploring the impact of substituents (–H, –CH3, –OCH3, and –NO2) in linkers and the length of the linkers (phenyl and biphenyl) on the structural and electronic properties. Our findings reveal the substantial impact of substituents on band edge positions, enabling tunable electronic properties. Charge analysis results show that electron-withdrawing groups significantly enhance charge separation by creating internal electric fields, thereby promoting efficient electron–hole separation. Thermodynamic analysis further highlights that the modulation of electron-withdrawing groups can generate distinct active sites for HER and the OER, which otherwise happens in similar sites in electron-donating groups, thus making the redox reactions thermodynamically spontaneous. Subsequently, we propose a biphenyl linker with a nitro group as an electron-withdrawing substituent that possesses optimal band edge positions for photocatalytic water splitting under minimal external bias. Moreover, the study reveals that an optimal balance between the number and the strength of electron-withdrawing substitution is important for HER and OER feasibility. Our findings demonstrate a rational strategy for tuning the electronic properties and hence modulating the OER and HER performance in β-ketoenamine-based COFs as metal-free photocatalytic materials.